Schistosomiasis is caused by parasites of the Schistosoma genus, and Schistosoma mansoni is the only species found in South America. The disease causes significant human morbidity and mortality in endemic areas worldwide. The main control strategy involves mass-treating infected human populations with the drug Praziquantel. However, this methodology is problematic given that parasite resistance to Praziquantel has been reported from a number of endemic areas. The long-term objective of this project is to identify Schistosoma mansoni genes involved in resistance to Praziquantel treatment. The objectives of Specific Aim #1 are to 1) determine natural variation of S. mansoni sensitivity to Praziquantel treatment, 2) isolate resistant strains and 3) compare genetic profiles of resistant versus susceptible strains. The objectives of Specific Aim #2 are to 1) identify genes involved in the resistant phenotype by comparing the transcriptome of susceptible versus resistant worms, and 2) modulate the resistance phenotype by controlling gene- expression levels. The proposed work will be performed using parasite eggs obtained from the feces of infected individuals. Parasite susceptibility to Praziquantel will be assessed by determining the amount of drug necessary to kill 50% of the infecting parasites (ED50) in a mouse model. In addition, this approach will allow us to identify resistant isolates. Parasites will be genotyped using polymorphic microsatellite loci already developed by the collaborating groups. We predict that resistant worms will demonstrate different gene-transcription profiles relative to susceptible individuals. To identify the genes responsible for these differences, Serial Analysis of Gene Expression (SAGE) will be employed. Identified genes will be further investigated in both resistant and susceptible worms by altering gene-expression patterns; RNA interference (RNAi) will be used to down-regulate gene expression, whereas plasmid constructs (containing the genes of interest) will used to up-regulate expression patterns. By utilizing these methods and techniques, we expect to clarify the mechanism(s) underlying drug resistance in Schistosoma mansoni, and to propose novel treatments and control measures based on our results. This research will be done primarily in Brazil at the Centra de Pesquisas Rene Rachou - FIOCRUZ in collaboration with Guilherme Oliveira as an extension of NIH grant # 2R01AI042768-05A2.